United have only recorded 10 points from their opening seven matches in the Premier League, and tension has threatened to boil over behind the scenes after disagreements with a number of players including Paul Pogba.
The Portuguese has also been heavily criticised for his team's style of play and according to The Sun, it could all contribute to the club and Mourinho parting company during the upcoming international break.
The report claims that a home defeat to Newcastle United at the weekend would lead to United's hierarchy seriously considering the departure of their manager.
United face crucial games with Chelsea and Juventus in the coming weeks, and defeats in those matches could seriously hinder the club's chances of gaining a top-four place in the Premier League and qualifying for the knockout stages of the Champions League.
United are currently on a four-match winless streak in all competitions.